The only way a parasite can get into a tank it has to be introduced, either through, plants, new fish but mainly through the water supply.

I suggest you do a Google on them also try and catch some and take it to a good LFS for advice.
' Keith

Are you sure it's a planaria? Can you take a picture?

If nothing in your tank has changed in a long time, I would think that it's a harmless detrivore whose population has just been too small for you to notice. You only found the one? If you find more, it's likely a sign that you're feeding too much. So long as they aren't all over the place and your fish aren't dying off, it's nothing to worry about. :) And yes, it is quite normal. A small population of detrivores is beneficial to your tank, like copepods in a saltwater tank, if you know about those. They also make great snacks for most fish. I had a mosquito fish who used to live off of nematodes by his own preference. He refused flake food because he was always already well-fed, and he was still very healthy. (I did manage to get a few flakes down him now and again.) Most people's problem with these guys is that they're unsightly, but they only become that way if there's a problem with how clean you're keeping the tank. Your betta may well eat them; it really depends on the betta. Maybe it's already been helping control the population for you without you ever knowing it.

There is the chance that it could be a parasite though. I don't recognize that description. Chances are, though, if it's not attached to a host, it's not a parasite. I would remove it (if you haven't already) and do as Keith suggested. Better safe than sorry. :)

EEEEw ! I have this very same problem, it groses me out! mine look like tiny sperm worms, NASTY! they are like to tiny for me to catch though, but believe me i try. When i trick something in the water to swoop them up to see if i cought it i cant see them, like they hide from me, he he just kiding but i do try my best to get rid of them cuz they are sick! have you heard or delt with these types of worms?

Surfer:
You have been doing some research! This post was really old!
Oh well...
If it is indeed a planaria, I have some good info for you, but if you can catch one please take it to a fish shop for definate id (preferably one that has a very very knowledgeable staff).

I was told by our university fish and wildlife department that planaria in fishtanks are absolutely normal especially if the tank has elevated nitrogen levels. They aren't usually dangerous to people or fish but be sure to wash your hands anyway. My betta ate mine, and I was informed that it is alright for them to eat planaria, in fact it makes a great afternoon snack.

If you have lots of these guys, I'd have a professional check out your tank. Either you have a different kind of wiggly thing, or there is something really odd about your tank chemistry. Planaria are normal, but not necessarily present in every tank, and not in super high numbers. To verify a planaria look for a flat, triangle shaped head, and maybe spots that look like eyes on the top of the head. They can detect the presence of light (and don't like it) but can't really see through these eye spots. Mine was completely white and I could see through it enough to detect a tubular system, probably digestive.

Good luck, I hope everything works out!
lexie